1. Field of the Invention
This invention relates to a short fiber-reinforced rubber composition as well as a pneumatic radial tire using the same, and more particularly to a short fiber-reinforced rubber composition having excellent workability in industrially mass production scale, durability and anisotropy and a pneumatic radial tire simultaneously establishing steering stability and ride comfort and having a low fuel consumption by reinforcing a sidewall portion with such a short fiber-reinforced rubber composition to enhance side stiffness.
2. Description of Related Art
There have been known various short fiber-reinforced rubber compositions having improved modulus, strength and the like by dispersing short fibers into diene-based rubber such as natural rubber or the like. They have been manufactured by a method wherein short organic fibers such as nylon, polyester, vinylon or the like are compounded with a rubber elastomer and then cured, if necessary.
And also, in order to improve the strength, elongation and so on of such a short fiber-reinforced rubber composition, there has been proposed a fiber-reinforced elastic body formed by making an average fiber size of the short fiber such as nylon or the like small and reinforcing with such fine fibers. The production of such an elastic body is carried out by a method wherein curable rubber, nylon fibers and a binder are melt-kneaded, for example, at a temperature higher than a melting point of nylon and the resulting kneaded mass is extruded at a temperature higher than the melting point of nylon and further subjected to drawing or rolling to obtain a fiber-reinforced rubber and another curable rubber and a curing agent are compounded therewith and then cured (JP-A-58-79037, JP-A-59-43041, JP-A-61-225858). The tensile strength, modulus and the like are certainly improved by this method, but since the fiber-reinforced rubber has a structure that fine fibers such as nylon or the like are dispersed in the curable rubber, there is a problem that the pelletization is very difficult. And also, when a great amount of nylon is compounded, the fiber-reinforced rubber becomes very hard, so that it is required to prolong the kneading time or to soften the fiber-reinforced rubber by warming before the compounding with another curable rubber and the like, which comes into problem in the productivity.
As a countermeasure for solving this problem, there is proposed a fiber-reinforced elastic body formed by dispersing fine fibers of thermoplastic polyamide into a matrix of polyolefin and diene-based elastomer and kneading the resulting thermoplastic composition with another diene-based elastomer (JP-A-7-278360).
When the fiber-reinforced elastic body disclosed in JP-A-7-278360 is compared with the conventional fiber-reinforced elastic bodies, the productivity in a laboratory scale and the modulus and fatigue resistance are improved, but the workability in the industrially mass production scale is poor. And also, such a fiber-reinforced elastic body is not necessarily sufficient when it is demanded to provide a higher reinforcing effect in a particular direction as in the reinforcement for a sidewall portion of a tire or the like because the difference between orienting direction of the fiber and direction perpendicular thereto is little.
In usual small-size radial tires for passenger cars, motorcycles and the like, in-plane bending rigidity of a sidewall portion comprising a thin sidewall rubber is particularly small as compared with that of a tread portion comprising a belt of various structures and a thick tread rubber, so that it is difficult to improve the steering stability of the tire, which is dependent upon side stiffness in the tire, without sacrificing the other tire performances such as ride comfort, durability and the like.
In order to directly increase the stiffness of the sidewall portion, there has been proposed a method wherein a fiber-reinforced member having a tire fabric structure of twisted yarns is laid in the sidewall portion adjacent to a carcass ply. In this case, however, the fiber-reinforced member can not follow to a large extension deformation of the sidewall portion at tire-building and vulcanization steps, and it is difficult to manufacture the tire using such a fiber-reinforced member by the conventional production method, and the flexibility of the sidewall portion is extremely lost in a tire product to increase a possibility of losing characteristics inherent to the radial tire. For this end, such a method has not yet been put to practical use.
As a countermeasure for solving the above problem, there have been made various proposals of using high modulus, low heat-shrinkable cords as a carcass cord to increase the stiffness of the sidewall portion and decreasing heat shrinkage in the vulcanization to stabilize the shape of the sidewall portion. In the radial tire for passenger cars, there has recently been proposed an attempt that a usual belt layer of steel cords is combined with an additional belt layer of various organic fiber cords or properties of a coating rubber for these layers are changed to thereby improve the steering stability.
In the above conventional techniques, when the high modulus, low heat-shrinkable cords are used as a carcass cord, the tensile rigidity in the radial direction of the tire coincident with the cord extending direction can effectively be increased by such a high modulus of the cord, but the in-plane bending rigidity of the sidewall portion and the rigidity in the circumferential direction of the tire perpendicular to the radial direction can not directly be increased. And also, the heat shrinking deformation of the tire is certainly decreased by using the low heat-shrinkable cords, but there can not be controlled irregularities of the sidewall portion in the tire under an inflation of an internal pressure resulted from the scattering of an end count (number of cords arranged per constant width) or the scattering of thickness of the sidewall portion in the manufacture of the tire. Therefore, the use of the above cord is not an effective means and can not develop satisfactory effects. On the other hand, when the steel cord belt layer is combined with the additional organic fiber reinforced belt layer using the organic fiber cords such as nylon, polyester or the like, or when the properties of the coating rubber for these belt layers are changed, it is possible to improve the steering stability of the tire resulted from friction force of the tread portion contacting with ground, but the effect of improving the steering stability can not be obtained when the tire is forcedly subjected to input directly deforming the sidewall portion during the running of the vehicle such as slalom running or the like.
Lately, it is advanced to reduce the rolling resistance of the tire from a demand for low fuel consumption in the vehicle, and it tends to more thin the thickness of the carcass ply in the tire for reducing the tire weight, which are difficult to attain only in the above conventional techniques.
Under the above situations, it is an object of the invention to provide a short fiber-reinforced rubber composition having excellent workability in industrially mass production scale, durability and anisotropy.
It is another object of the invention to provide a pneumatic radial tire simultaneously establishing steering stability and ride comfort and having a low fuel consumption by increasing stiffness of a sidewall portion without damaging characteristics inherent to the radial tire such as ride comfort, durability and the like and complicating the production method, and capable of reducing the rolling resistance.
According to a first aspect of the invention, there is the provision of a short fiber-reinforced rubber composition comprising a master batch (A) comprised of (a) a first diene-based elastomer, (b) a polyolefin having a melting point of 100-150xc2x0 C. and (c) short fibers of a thermoplastic polymer having amide group in its main chain, wherein the component (c) is dispersed into a matrix formed by dispersing less than 100 parts by weight of the component (b) into 100 parts by weight of a continuous phase of the component (a), and a second diene-based elastomer (B) kneaded with the master batch (A), in which an amount of the component (c) is 5-30 parts by weight based on 100 parts by weight in total of the first diene-based elastomer and the second diene-based elastomer.
In preferred embodiments of the first aspect of the invention, the component (c) is chemically bonded to both the components (a) and (b), and the component (c) has an average diameter of 0.05-1.0 xcexcm and a ratio of average length to average diameter of 10-2000, and an amount of the component (b) occupied in 100 parts by weight of the continuous phase of the component (a) is less than 85 parts by weight, preferably less than 65 parts by weight, more particularly less than 45 parts by weight, and an amount of the component (c) occupied in 100 parts by weight of the continuous phase of the component (a) is not less than 50 parts by weight but less than 110 parts by weight, preferably not less than 50 parts by weight but less than 100 parts by weight.
According to a second aspect of the invention, there is the provision of a pneumatic radial tire comprising a pair of bead portions, a pair of sidewall portions, a tread portion, a radial carcass toroidally extending between a pair of bead cores embedded in the bead portions and wound around the bead core from an inside of the tire toward an outside thereof to form a turnup portion, a belt disposed on an outer surface of a crown portion of the carcass in a radial direction of the tire, and a bead filler disposed above the bead core between the carcass and its turnup portion, wherein a rubber sheet having a thickness of 0.5-2.0 mm is made of a short fiber-reinforced rubber composition as claimed in the first aspect of the invention and inserted in a zone of the sidewall portion so as to at least contact an end with an upper end of the bead filler and make an orienting direction of the short fibers within 0xc2x0xc2x145xc2x0 with respect to a circumferential direction of the tire.
In preferred embodiments of the second aspect of the invention, the rubber sheet is inserted in a height zone of the sidewall portion corresponding to not more than 95%, preferably not more than 80% of a carcass height between a base line at a bottom of the bead core and a center position of a carcass line, and the orienting direction of the short fibers is within 0xc2x0xc2x115xc2x0 with respect to a circumferential direction of the tire.
According to a third aspect of the invention, there is the provision of a pneumatic radial tire comprising a pair of bead portions, a pair of sidewall portions, a tread portion, a radial carcass toroidally extending between a pair of bead cores embedded in the bead portions and wound around the bead core from an inside of the tire toward an outside thereof to form a turnup portion, a belt disposed on an outer surface of a crown portion of the carcass in a radial direction of the tire, and a bead filler disposed above the bead core between the carcass and its turnup portion, wherein a rubber sheet having a thickness of 0.5-2.0 mm is made of a short fiber-reinforced rubber composition as claimed in the first aspect of the invention and inserted in a zone of the sidewall portion so as to have an overlapping margin of at least 0.5 mm with a turnup end of the carcass and make an orienting direction of the short fibers within 0xc2x0xc2x145xc2x0 with respect to a circumferential direction of the tire.
In preferred embodiments of the third aspect of the invention, the rubber sheet is inserted in a height zone of the sidewall portion corresponding to not more than 95%, preferably not more than 80% of a carcass height between a base line at a bottom of the bead core and a center position of a carcass line, and the orienting direction of the short fibers is within 0xc2x0xc2x115xc2x0 with respect to a circumferential direction of the tire.